Global climate change: status and prospects

The world’s climate is changing; this is well documented. Many climate parameters reflect the changes we see all over the world today: the atmosphere and the oceans are getting warmer, CO2 levels in the atmosphere are increasing, sea level is rising, and snow and ice are melting away. The earth’s climate has always varied, and there have always been physical explanations for these variations. Over the last 30 to 40 years, however, the climate has changed more rapidly than can be explained by natural causes.

Status

Figure: Working Group I Contribution to the IPCC Fifth Assessment Report

Figure: Working Group I Contribution to the IPCC Fifth Assessment Report

According to the most recent report from Working Group I of the United Nations’ Intergovernmental Panel on Climate Change (IPCC)[1]the average temperature on our planet has increased by 0.85°C in the past century, between 1880 and 2012, and the temperature has increased by 0.6°C since 1951. Each of the last three decades has been warmer than any past decade since 1850. In the northern hemisphere, the period between 1983 and 2012 was probably the warmest 30-year period in the past 1 400 years. This temperature increase is observed all over the world, but is largest in the northern hemisphere. The changes and trends in precipitation are less certain. Nonetheless, there is evidence showing that regions where extreme precipitation events have become more frequent outnumber those where they have become less frequent. Both North America and Europe have experienced more frequent or intense precipitation events.

Several long-term changes in climate have been observed, and they differ between continents, oceans and on a regional scale. Among these changes are higher temperatures and less ice in the Arctic, and alterations in total precipitation, sea salinity and wind patterns, occurrence of extreme weather such as drought, heavy rainfall, heat waves and intensified tropical cyclones.

IPCC is now quite certain that the net effect of human activity since 1750 has contributed towards this global warming, and that anthropogenic drivers have contributed between 0.5°C and 1.3°C of the increase seen from 1951 to 2010 (including the cooling effect of aerosols). Changes in extrinsic factors (sun cycle, volcanic activity, etc) and natural variability (slow responses of oceans and glaciers, for example) account for between ‑0.1°C and 0.1°C.

Future scenarios

Climate models

Climate models are highly advanced computer programs based on mathematical formulas describing the physical processes that influence the earth’s climate. These programs calculate what will characterise the climate of the future by combining the climate models with difference scenarios for emission of greenhouse gases

No climate model is perfect. Nonetheless, climate models have developed significantly in recent years, and continuous effort goes into further improvement, for example concerning regional differences and mechanisms in the climate system that are unique to polar areas.

Some parts of the climate system, such as clouds and particles in the atmosphere, remain exceedingly difficult to describe in mathematical formulas. Different models are constructed in different ways and include different components: they can yield somewhat different results. There is also uncertainty concerning future emissions of greenhouse gases and particles, which influences the certainty of the climate scenarios.

So far, climate models have not been able to project the rapid changes in the Arctic, although they have shown warming trends. The ice is melting faster that the models predict.

All currently available climate models predict that warming will continue and accelerate. Future emission of greenhouse gases is a major factor determining how much the earth warms during the 21st century.

The most recent report from IPCC’s Working Group I[1] states that regardless of the representative concentration pathway (RCP)the global average temperature will increase by more than 1.5°C relative to the period 1850-1900, in the worst case by as much as 2.0°C. An increase of 0.3°C to 0.7°C is expected between the twenty-year periods 1986-2005 and 2016-2035. The models show that continued global warming is inevitable even after the end of this century. Most models show that warming in the Arctic will continue to be greater than in other areas. It is also estimated that extreme temperatures will be more common worldwide, and that heat waves are very likely to be more frequent and last longer.

Representative concentration pathway

The Fifth Assessment Report from IPCC often refers to four concentration scenarios: Representative Concentration Pathways (RCPs). These scenarios are not assumptions about what the future holds. They describe which changes we can expect in the climate depending on the magnitude of greenhouse gas emissions the years to come.

The RCPs were selected to represent the entire spectrum of possible future emissions of greenhouse gases, and every one of these scenarios leads to an increase in temperature.